1. Field of the Invention
The present arrangement relates to fiber optic cables. More particularly, the present arrangement relates to a fiber optic cable with a modified construction for cordage or tactical applications.
2. Description of the Related Art
Cordage and tactical applications for fiber optic cables are typically required to meet very stringent testing requirements, such as being able to operate under extreme temperatures, have good tensile strength, higher resistance to abrasion and crushing, etc. However, at the same time tactical fiber optic cables also need to maintain a certain level of flexibility, so as to be wound and unwound from relatively small drums for fast and easy field deployment.
For example, a typical tactical fiber optic cable may be constructed as a tight buffer optical fiber(s), surrounded by longitudinal or slightly stranded aramid fibers and enclosed within an outer jacket of polyurethane. The tight buffer optical fiber is generally a more protected fiber than normal UV optical fibers. The aramid fibers provide strength to the cable, such as tensile strength, and the polyurethane jacket provides a tough but flexible outer casing that can endure severe temperatures. See prior art FIG. 1.
However, such a cable, although flexible, still has certain drawbacks associated with its ability to wind and unwind around tight cable drums, such as for application requiring highly portable fiber drums. Because of the properties of the polyurethane under partial pressure extrusion, including its melt-flow properties (and which has no measured shrinkage after the jacket is removed an exposed to 110° C. for 2 hrs), the jacket minutely encapsulates some of the layer of aramid fibers/strength members causing the outer portion of the strength layer to “weld” into the inside diameter of the polyurethane jacket as shown for example in prior art FIG. 2. In other words, the aramid and jacket combination, at least in part forms an aramid reinforced polymer at their interface.
This welding of the strength layer to the inside of the jacket partially fuses the two layers, reducing flexibility, particularly when the cable is turned around a non-standard tight or reduced diameter drum, tent post, or mandrel during testing. In an ideal non-welded situation the surface of the jacket (particularly at the inside portion of the tightest bending) is able to stretch and the aramid fibers therein may re-position so that the jacket and strength fibers do not transfer the bending stresses down/up onto the fibers therein. However, as illustrated in prior art FIG. 3, when the aramid fibers are welded into the jacket as shown in FIG. 2 above, and when the cable is bent around a drum or mandrel, the shorter path (inner surface of the jacket against the drum) must absorb all of the shortening since aramid-welded polymer cannot stretch. This results in the inner radius jacket having to collapse on itself in an accordion fashion and the inner uncoupled constituents of strength yarn filaments or aramid fibers, optical fibers, tight buffered optical fibers, subunits or fillers being forced to adapt or collapsed to a reduced longitudinal space or to be longitudinally “crushed” into a sine-wave shape. Additionally, the strength yarn filaments or fibers welded in along the top surface of the bend (away from the drum or mandrel) cannot reposition and are pulled down on the upper surface of the fiber in the middle of the cable. The distorted jacket and welded aramid fiber combination pushes in towards the strength filaments or fibers, optical fibers, tight buffered optical fibers, subunits or fillers in the center causing either unacceptable levels of attenuation of even outright failure of the cable.
In an exemplary calculation using a tactical cable with an outer diameter (OD) of 0.310″ being wrapped around a 3″ mandrel the following equation shows the approximate crushing percentage (length differential caused by bending around the mandrel) that must be entirely absorbed on the inner diameter of the bent cable when the welded aramid jacket cannot reposition or stretch along the outer diameter.π(3+0.310)·π(3)/π(3)=0.310/3=0.1033=10.33%[(π*diameter of outside bend)·(π*diameter of inside bend)/(π*mandrel diameter)]
Using the same size OD cable 0.310 around a 2″ mandrelπ(2+0.310)·π(2)/π(2)=0.310/2=0.155=15.5%
Likewise, in an exemplary calculation using a tactical cable with an outer diameter (OD) of 0.175″ being wrapped around a 2.5″ mandrel the following equation shows the approximate crushing percentage that must be entirely absorbed on the inner diameter of the bent cable when the welded aramid jacket cannot reposition or stretch along the outer diameter.π(2.5+0.175″)·π(2.5)/π(2.5)=0.175/2.5=0.07=7%
Using the same site OD cable 0.175 around a 2″ mandrelπ(2+0.175)·π(2)/π(2)=0.175/2=0.0875=8.75%
Using the same size OD cable 0.175 around a 1″ mandrelπ(1+0.175)·π(1)/π(1)=0.175/1=0.175=17.5%
As such, in a cable according to the prior art, with the aramid strength fibers welded into the jacket, and with the outer surface of the jacket on the bend unable to stretch, the two surfaces share the amount that must be absorbed by the bend, the inner surface of the bend must absorb roughly 50% of the approximated 10%-15% length differential (or 5% to 7.5%) as shown in FIG. 3.